Fixed Assertion failures from isDone() method call on future

Change-Id: I39693e789222f39a263d365546fda169e1182645
Reviewed-on: http://review.couchbase.org/27885
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Deepti Dawar <deepti.dawar@globallogic.com>